1. Field of the Invention
The invention relates generally to the field of photography, and particularly to camera apparatus for preventing double exposure when a film cassette containing a partially or fully exposed filmstrip is loaded into the camera apparatus.
2. Description of the Prior Art
Generally speaking, commercially available 35 mm film cassettes such as manufactured by Eastman Kodak Co. and Fuji Photo Co., Ltd. comprise a hollow cylindrical shell having an integral throat or lipped portion which extends tangentially from the shell. A film spool on which the filmstrip is wound is freely rotatable within the cassette shell. The filmstrip has a leading section, commonly referred to as a "film leader", which protrudes from a light-trapped slit in the throat portion to the outside of the cassette shell. Typically, the protruding leader extends 2-3 inches in length.
In conventional 35 mm film cassettes, one end of the film spool on which the filmstrip is wound has a short axial extension which projects from the cassette shell, enabling the spool to be turned by hand. If the spool is manually rotated initially in an unwinding direction, the film convolutions inside the cassette shell will tend to expand radially since the inner end of the filmstrip is attached to the spool, and the film leader protruding from the slit will remain stationary. The film convolutions can expand radially until a non-slipping relation is established between the outermost convolution and the inner curved wall of the cassette shell. Once this non-slipping relation exists, there is a binding effect between the outermost convolution and the cassette shell which prevents further rotation of the spool in the unwinding direction. Thus, rotation of the spool in the unwinding direction cannot serve to advance the filmstrip out of the shell, and it is necessary in the typical 35 mm camera to engage the protruding leader to draw the filmstrip out of the shell.
A 35 mm film cassette has been proposed which, unlike conventional film cassettes, can be operated to automatically advance the filmstrip out of the cassette shell by rotation the film spool in the unwinding direction. Moreover, the film leader is non-protruding, i.e. it is located entirely within the cassette shell. Specifically, in U.S. Pat. No. 4,423,943, granted Jan. 3, 1984, there is disclosed a film cassette wherein the outermost convolution of the film roll wound on the film spool is radially constrained by respective circumferential lips of two axially spaced flanges of the spool to prevent the outermost convolution from contacting an inner wall of the cassette shell. The trailing end of the filmstrip is secured to the film spool, and the leading end of the filmstrip is reduced in width to allow it to produce from between the circumferential lips and rest against the shell wall. During unwinding rotation of the film spool, the leading end of the filmstrip is advanced to and through a film passageway opening in order to exit the cassette shell. As a result, all that is needed to advance the filmstrip out of the cassette shell is to rotate the film spool in the unwinding direction. However, according to U.S. Pat. No. 4,423,943, the film cassette is intended to be loaded in a camera only after the non-protruding leader is advanced to the outside of the cassette shell. In the patent, it is suggested that one manually rotate the film spool relative to the cassette shell until the film leader can be manually grasped and attached to a film advancing device in the camera. Thus, the camera is not used to rotate the film spool to advance the filmstrip from the cassette shell.
3. The Cross-Referenced Application
Like the type of film cassette disclosed in U.S. Pat. No. 4,423,923, cross-referenced application Ser. No. 221,995 discloses a film cassette that contain a filmstrip, including a non-protruding leader, which is automatically advanced to the outside of the cassette shell in response to rotation of the film spool in an unwinding direction. However, unlike the patent, the cross-referenced application discloses a camera having a motor-driven film transport (rather than manual means) for rotating the film spool to propel the filmstrip from the cassette shell.